Add Google Plus snippet bot to user agents list

[Google's snippet crawler](https://developers.google.com/+/web/snippet/) fetches page information for links shared on Google Plus. It does does not seem to support *escape_fragement*.

Unlike `facebookexternalhit` and `linkedinbot` the User Agent does not have a friendly bot name (`Google (+https://developers.google.com/+/web/snippet/)` could be confused with googlebot, et al.), so a uniquely identifiable component of the full value is used: `developers.google.com/+/web/snippet/`.

Let me know if there are any questions! It'd be great to see these changes incorporated to make social sharing nearly thoughtless.